1. Field of the Invention
The present invention relates to burst mode limiter-amplifier devices, and in particular to limiter-amplifier devices for variable mode use with burst mode and continuous optical communications.
2. Background of the Technology
In the existing art, most optical receivers and optical communications involve continuous mode communication. One new method for optically communicating involves burst mode communication. Burst mode communication is useful, for example, for point to multipoint communication, such as occurs when a single operator is linked to many users. In the prior art, for example, many users are connected to a single operator using fiberoptic lines that are split among the users: typically, for example, one fiber is split to local fibers that are in turn coupled to optical users. In order to prevent interference among the users, every user transmits using a different carrier. Thus, at multiple times, the user communication is quiet, then the user starts a burst of transmission, and then the user shuts down again, waiting for a next period for transmission.
The difficulties of receiving and distinguishing among receivers is exacerbated by typically large variations in the magnitude of power of the bursts among different users. FIG. 1 is a graphical representation demonstrating a low power burst following a high power burst, as typically occurs in a burst communication, as is known in the art. As shown in the example of FIG. 1, the power difference between two successive bursts can be about 15-25 dB. Also, as shown in FIG. 1, the high power burst 1 can raise the average power level of a sequentially following low power burst 2, which decays slowly over time. There are a number of reasons for the slow decay in the power of a burst, including the following: 1) finite bandwidth of the circuits before and after the TIA; 2) turn off time of the laser diode (LD) and LD driver; and/or 3) turn off time of the P-I-N avalanche photodiode (APD). The decay time, together with the physical (PHY) definitions (i.e., guard time, preamble length) determine if there is a need to treat this phenomenon.
These phenomena make the receiver implementation much more difficult, and the usual architecture of the prior art for continuous mode implementations, shown in FIG. 2, fails to operate properly for burst mode communications. As shown in FIG. 2, a typical optical receiver 20 of the prior art, which identifies signals by virtue of a technique known as alternating current (AC) coupling, includes three primary components. The first component is a transimpedence amplifier (TIA) 22. The TIA 20 receives the typically weak signal output from the optical detector coupled, for example, to an optical fiber line, and amplifies the signal. Next, the device must discriminate between a high level and a low level signal that is received, the optical signal typically including xe2x80x9con/off keying,xe2x80x9d which consists of transmitted xe2x80x9conxe2x80x9d signals (also referred to as xe2x80x9conesxe2x80x9d) and xe2x80x9coffxe2x80x9d signals (also referred to as xe2x80x9czerosxe2x80x9d). With an optical device, the transmitted xe2x80x9conxe2x80x9d signal (xe2x80x9conexe2x80x9d) is a pulse of light, while the transmitted xe2x80x9coffxe2x80x9d signal (xe2x80x9czeroxe2x80x9d) is the non-transmittal of light. The current produced by the received light is amplified by the TIA 22, and a capacitor 23 filters the noise from the amplified signal (i.e., removes the direct current (DC) portion of the signal).
Since the data is transmitted in bursts, a problem arises in that the receiver must receive and distinguish bursts of data. The receiver must recognize each transmitter that transmits data, and the receiver typically must estimate the power of the data to distinguish among bursts. In order to make this determination, the receiver must acquire the signal for the data burst within a short time period at the beginning of the burst.
In the prior art, continuous mode transmission and reception has typically been used with two station transmitters, for which data are continuously transmittedxe2x80x94no stopping and starting of data occurs, as is the case with burst mode transmission. As a result, in prior art continuous transmission, it has not mattered how long it takes for the receiver to acquire the signal.
There remain a number of unmet needs in the prior art for using limiter-amplifiers for burst mode communications. One problem with the limiter-amplifiers of the prior art is that filtering the signal takes time, which slows signal transmission. Another problem with the prior art is that limiter-amplifier devices cannot be readily or easily adapted to different types of signals (e.g., differing acquisition periods).
The present invention includes a limiter-amplifier device usable for burst mode communications with variable acquisition periods. In one embodiment, the device includes a resistor bypassable via, for example, a switching device (e.g., a single pole switch; a transistor; or other devices known in the art; hereinafter referred to interchangeably as a xe2x80x9cswitchxe2x80x9d), a capacitor, and a limiter-amplifier in series within the device. In operation, in a first phase (reset), the capacitor is discharged to ground via a pair of switches, each of which is operable so as to connect a side of the capacitor to ground. In a second phase, the resistor is bypassed (via, for example, a third switch), and the capacitor is charged to a threshold voltage between the received signal and ground, via opening of a first one of the switches used for discharging the capacitor.
The third phase includes a fast tracking mode and a slow tracking mode. In the fast tracking mode, the resistor is bypassed, and the capacitor is placed in series with the limiter-amplifier via opening of the first capacitor discharge switch and via moving of the second switch to a position coupling the capacitor to the limiter-amplifier. In the slow tracking mode, the resistor is not bypassed, by, for example, leaving the third switch open, and the resistor and capacitor thus are in series with the limiter-amplifier.
In a second embodiment, the resistor is so placed that less switching functions are needed to bypass the resistor. In this embodiment, single pole switches, for example, are situated on either side of the capacitor, so as to allow discharge to ground by closing of these switches in the first phase. The resistor is positioned in series between the second side of the capacitor and the limiter-amplifier. In the second phase, the first capacitor side switch is opened, and the second capacitor side switch is left closed to ground, allowing fast charging of the capacitor. In the third phase, both switches are opened, such that the capacitor is in series with the resistor and the limiter-amplifier. In this embodiment, additional resistors are optionally used in series with the switches to avoid exceeding current limitations for the switches.
The present invention overcomes problems of the prior art relating to input from the TIA causing difficulties for the limiter-amplifier, such as the following: 1) the input has high DC level, (Common mode, in the case of differential amplifier), which can change from burst to burst; and 2) the non-linearity causes a duty-cycle distortion, especially for large signals. The present invention solves the first of these difficulties by providing two or more time constants via variable resistances in the circuit, a short time constant for acquisition during the preamble reception and longer time constant for tracking. Moving from one time constant to the other is controlled in various embodiments of the present invention via use of switches in the device.
Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the following or upon learning by practice of the invention.